1. Field of the Invention
The present invention relates to a transmission apparatus which is equipped in wire transmission apparatus used in an optical subscriber transmission system or the like and adapted to send and receive audio information or the like through the encryption/decryption processes.
2. Description of the Prior Art
FIG. 1 is a block diagram showing the transmission system which includes a transmission apparatus equipped with a conventional secret communication apparatus. In the figure, symbols 1a through 1n denote network terminating equipments installed on the part of subscribers, 2 is a terminal equipment installed on the part of an exchange office, and 3 is an optical fiber transmission line which connects the network terminating equipments 1a-1n to the terminal equipment 2 in the form of passive double star (PDS) system in which a single optical fiber cable is branched midway into a plurality of subscriber's lines.
Each of the network terminating equipments 1a-1n incorporates a transmission path interface circuit 11 which performs conversion between the optical signal transmitted over the optical fiber transmission line 3 and the electrical signal processed inside the network terminating equipment, 12 is a scrambler which encrypts outgoing data and transfers the resulting data to the interface circuit 11, and 13 is a descrambler which decrypts incoming data received by the interface circuit 11 thereby to restore the original data. The terminal equipment 2 incorporates a transmission path interface circuit 21 which is similar to the interface circuit 11 used in the network terminating equipment, 22 is a scrambler similar to the one 12, and 23 is a descrambler similar to the one 13.
Next, the operation will be explained. The optical subscriber transmission system shown in FIG. 1 employs the PDS cable system, and therefore the same optical signal is delivered to all subscriber's network terminating equipments 1a-1n. On this account, the network terminating equipments 1a-1n and terminal equipment 2 not only perform signal separation based on time slots, but they perform data encryption and decryption by means of the scramblers 12 and 22 and descramblers 13 and 23 thereby to prevent the leakage of communication data from one subscriber to another.
Transmission data to be sent from the subscriber is fed to the scrambler 12 in the subscriber's network terminating equipment so that it is encrypted, and the encrypted data is fed to the transmission path interface circuit 11. The interface circuit 11 converts the encrypted electrical transmission data into an optical signal, and sends the signal over the optical fiber transmission line 3. In the exchange office, the signal is received by the transmission path interface circuit 21 in the terminal equipment 2, by which the optical signal is converted to an electrical signal and fed to the descrambler 23. The descrambler 23 decrypts the received data thereby to restore the original data.
Data to be sent from the exchange office to a subscriber is encrypted by the scrambler 22 in the terminal equipment 2 and, after transmission, the received data is decrypted by the descrambler 13 in the network terminating equipment of the subscriber so that the original data is restored, in the same manner as mentioned above.
However, the conventional secret communication apparatus described above is deficient in that if the scrambling scheme used for the scramblers and descramblers in the system is known by the third party, transmitted messages can easily be exposed and transmitted data can easily be analyzed by the third party.